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Cathode Ray Tube (CRT) Direct View and Rear Projection TVs

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Technology Overview & Description

A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when a moving electron beam strikes a phosphorescent surface. There are three factors that limit the resolution on CRT display devices: screen dot pitch, electron beam size, and the bandwidth of the video amplifier. A typical CRT has a dot pitch around 0.8 - 0.9mm (much larger than a typical computer display). Lowering the dot pitch increases the display resolution, but increased dot pitch provides a brighter picture. Most CRT displays are configured to perform well with lots of ambient light, so dot pitch is typically higher.

Editor's Note: Dot Pitch Explained
Dot Pitch, or phosphor pitch, is a measurement indicating the diagonal distance between like-colored phosphor dots on a display screen. Measured in millimeters, the dot pitch is one of the principal characteristics that determine the quality of display monitors. The lower the number, the crisper the image. The dot pitch of color monitors for personal computers ranges from about 0.15 mm to 0.30 mm.

Rear projection TVs typically utilize 7" CRT guns, with some of the higher-end models using 9" guns. 7" guns can typically resolve about 700-800 lines of resolution. The high end 9" guns can do upwards of 900 lines. Typical direct view televisions deliver just over 600 lines of resolution. Most RPTVs have at least 30Mhz of video amplifier bandwidth, which is good for just under 720p or 1080i. Better models have upwards of 75Mhz. Most direct view televisions have 20Mhz video amplifiers, with some higher-end units extending above 30MHz.

CRT televisions receive video signals at the rate of 30 frames a second. Each frame of video contains about 480 lines of information. A single frame is projected on the screen line-by-line in two passes (each pass is called a "field"). On the first pass, the beam projects all of the odd numbered lines from 1-479 from top to bottom. On the second pass, it projects all of the even numbered lines from 2-480. It takes 1/30 of a second to complete both passes. This process is called interlacing. CRT type TVs need time to reset the electronic beam to the top of the screen so it can get ready to paint the next sequence of lines. To accomplish, they build in an interframe gap that equals about 45 lines. There is no picture information here. So the total lines per frame are 525 (480 + 45). Thus standard definition TV (SDTV) is often referred to as 480i (interlaced).

What's Next

Extinction! Front projection CRT-based systems are all but gone already. Rear projection CRT is soon to follow as soon as LCOS and DLP systems drop in price (LCD is already providing an easy way to get HDTV on the cheap). Samsung seems to be holding on, and they are one of the few. As rival flat screen and digital projection technologies drop in price and increase in quality, there will become less and less reason to pay for the bulk and power consumption of CRT displays. It is very likely that if you have a newborn, when he is a teenager he will look at you quizzically if you say the words "CRT" (and good luck explaining a record player).

CRT Direct View/Rear Projection Advantages

  • Among the clearest alternatives
  • Excellent color and contrast potential
  • Relatively inexpensive
  • Excellent life expectancy

CRT Direct View/Rear Projection Disadvantages

  • Heavy
  • Very deep (new models imprive on this, but units are still relatively bulky)
  • Did we mention heavy?
  • Analogue connectivity or D/A conversion of digital input connections
  • Potential for screen burn-in

sony_KV34XBR910.jpg mitsubishi_WS55813.jpg

Sony KV34XBR910 Direct View CRT (left); Mitsubishi WS-55813 Rear Projection CRT (right)

 

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